The invention relates to an optical component comprising a first element having a light-emission surface and a second element having a light-entrance surface, a bonding layer interconnecting said elements being situated between said surfaces.
The invention also relates to a method of manufacturing an optical component comprising a first element having a light-emission surface and a second element having a light-entrance surface, a bonding layer interconnecting said elements being situated between said surfaces.
Such an optical component and a method of manufacturing said component are disclosed in U.S. Pat. No. 5,074,683.
In the known method, two optical elements are interconnected by means of the bonding layer, a distance which is to be varied being provided by a spacer layer. Said spacer layer is provided only over a part of the first optical element, as a result of which an open space is formed between the first and the second optical element, which space is filled with a coupling mass having a refractive index which is essentially equal to the refractive index of the first optical element. The coupling mass is an epoxy, a gel or a comparable material.
The space between the surface of the first optical element and the surface of the second element is accurately adjustable by choosing the thickness of the spacer layer. The thickness of the spacer layer is typically 600 xcexcm. The known method is predominantly used to secure a fiber optic faceplate to sensors comprising charge-coupled devices (CCDs). The coupling mass is predominantly provided between the fiber optic faceplate and the active region of the sensor in order to obtain a satisfactory optical coupling without causing damage to the CCD sensor.
A drawback of the known method resides in that epoxies and gels generally shrink as a result of drying of the layer. If the epoxy or the gel is provided on the active part of a CCD sensor, the lifetime of the sensor is adversely affected by mechanical stresses. Wires establishing an electrical connection between the sensor and an envelope may become detached or, in the course of time, be subject to breakage.
A further, important drawback of the known coupling mass resides in that bubbles often develop in the epoxy and in the gel between the CCD sensor and the fiber optic faceplate, as a result of which the sensor becomes useless. Failure of such sensors at the end of the manufacturing process is very expensive.
It is an object of the invention to provide an optical component of the type described in the opening paragraph, which can be produced in a reliable and simple manner.
Another object of the invention is to provide a method of manufacturing an optical component of the type described in the opening paragraph, which method can be carried out in a reliable and simple manner, and which enables product failure to be reduced substantially.
As regards the optical component, this object is achieved in accordance with the invention in that the bonding layer is a transparent layer of paraffin.
Optical components, such as optical interconnects, are frequently used in data communication systems and in multi-chip modules. An optical interconnect may be a hybrid chip wherein GaAs is used for the photonics and Si is used for the electronics. The first element having a light-emission surface is, for example, a GaAs laser or light-emitting diode (LED), the second element having a light-entrance surface is, for example, a photodetector. The GaAs laser or the LED and the Si photodetector are coupled to each other by a transparent layer of paraffin, and bonding wires are frequently used for their final assembly in an envelope. The transparent layer of paraffin of the optical component in accordance with the invention is an excellent bonding layer and a medium having a good transmission in the wavelength range of visible light. As paraffin is comparatively soft with respect to epoxies or gels, mechanical stresses transferred to the elements, such as lasers or photodetectors, are very small. Problems regarding the lifetime of the elements caused by the bonding layer are precluded, as are the problems of the bonding wires becoming detached or being subject to breakage. The optical component can be manufactured by applying liquid paraffin to the light-emission surface of the first element and placing the second element with the light-entrance surface on said light-emission surface.
The optical component may have a capillary space between the light-emission surface and the light-entrance surface of the elements. Said capillary space is filled with the transparent layer of paraffin. The optical component can be obtained, for example, by joining the first element and the second element by bringing together their surfaces so as to form a capillary space, filling said capillary space by making it suck up liquid paraffin, cooling the paraffin and solidifying the paraffin so as to obtain a bonding layer of transparent paraffin in the capillary space.
An advantage of the transparent layer of paraffin is that the adhesion between the light-emission surface and the light-entrance surface of the elements is very good as a result of the Van der Waals"" forces. If the paraffin is present in a capillary space, the amount of material disappearing at the sides of the capillary space is very small, which can be partly attributed to the low vapor pressure of paraffin.
Paraffin is a mixture of aliphatic hydrocarbons. In the solid state, paraffin is a white to light yellow greasy paste. Dependent upon the composition of the mixture, the melting point of paraffin lies in the range between 45 and 65xc2x0 C. For practical applications, it is advantageous if the transparent paraffin layer is a solid at temperatures below 50xc2x0 C. If the elements are improperly attached to each other, for example improperly aligned, they can be readily detached by heating the paraffin layer in the capillary space to a temperature above the melting point. By virtue thereof, the number of optical components rejected during their manufacture is substantially reduced.
Advantageously, the degree of light scattering in the medium between the elements is low in order to minimize optical losses. A short distance between the optical elements is very favorable. Preferably, optical losses above 10% are precluded, so that the thickness chosen for the paraffin layer is maximally 200 xcexcm.
By minimizing the distance to the surface roughness of the surfaces, there is only a small variation in thickness of the layer and light scattering is virtually limited to Rayleigh scattering.
Favorably, reflections at surfaces are precluded as much as possible. By choosing the refractive index of the material of the bonding layer to be as close as possible to the refractive index of the elements, reflections of the light at the surface are reduced substantially. Paraffin has a favorable refractive index for visible light of typically 1.440-1.484, which is rather close to the refractive index of materials which are important in the semiconductor technology, such as quartz having a refractive index of 1.46. By virtue thereof, a quartz plate accommodating a bundle of optical fibers can be coupled substantially without reflections, via the paraffin layer, to an SiO2 passivation layer on the surface of a Si semiconductor detector.
Image pick-up devices, such as cameras, camcorders and digital cameras generally comprise an image sensor. The image sensor comprises an array of light-sensitive pixels. The light is converted per pixel into an electric signal by a solid state image device, for example a photodiode or a CCD.
Apart from a light-receiving image sensor, a pick-up device generally also comprises a plate accommodating a bundle of fibers which open into the light-emission surface. The transparent paraffin layer extends between the plate and the light-entrance surface of the image sensor. The plate is accurately fixed with respect to the image sensor by the paraffin. Light is incident on one or more pixels of the image sensor via an optical fiber. The amount of light per pixel is converted into an electric signal that is used to build up an image after electronic signal processing.
The object of the invention with regard to the method is achieved in accordance with the invention in that the first and the second element are fitted together by joining the surfaces so as to form a capillary space, which capillary space is filled by making it suck up liquid paraffin, the paraffin is cooled and solidified in order to obtain a bonding layer of transparent paraffin in the capillary space.
A very favorable effect of the method in accordance with the invention resides in that minimization of the surface energy between the elements causes the process wherein liquid paraffin is sucked up to continue until the capillary space is filled. Very surprisingly, it has been found that the paraffin layer remains transparent after solidification. By using this method, no, or hardly any, bubbles develop in the layer of paraffin. The method is very simple and inexpensive. Product failure in a late stage of the production process is precluded in that the elements can be readily detached by heating the paraffin to a temperature above the melting point. The method can then be repeated.
Favorably, the elements are subjected to pressure during the process wherein liquid paraffin is sucked up. As a result of said pressure, the elements cannot move with respect to each other, so that they remain more accurately positioned with respect to each other.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.